The way plugins are installed has changed. The correct command as above is now:
sudo bin/plugin install file:/path/to/plugin.zip
https://www.elastic.co/guide/en/elasticsearch/plugins/2.0/installation.html
However plugins now require a 'plugin-descriptor.properties' file to be included in the plugin zip alongside the .jar files
https://www.elastic.co/guide/en/elasticsearch/plugins/2.0/plugin-authors.html
https://github.com/elastic/elasticsearch/blob/master/dev-tools/src/main/resources/plugin-metadata/plugin-descriptor.properties
# Elasticsearch plugin descriptor file
# This file must exist as 'plugin-descriptor.properties' at
# the root directory of all plugins.
#
# A plugin can be 'site', 'jvm', or both.
#
### example site plugin for "foo":
#
# foo.zip <-- zip file for the plugin, with this structure:
# _site/ <-- the contents that will be served
# plugin-descriptor.properties <-- example contents below:
#
# site=true
# description=My cool plugin
# version=1.0
#
### example jvm plugin for "foo"
#
# foo.zip <-- zip file for the plugin, with this structure:
# <arbitrary name1>.jar <-- classes, resources, dependencies
# <arbitrary nameN>.jar <-- any number of jars
# plugin-descriptor.properties <-- example contents below:
#
# jvm=true
# classname=foo.bar.BazPlugin
# description=My cool plugin
# version=2.0.0-rc1
# elasticsearch.version=2.0
# java.version=1.7
#
### mandatory elements for all plugins:
#
# 'description': simple summary of the plugin
description=${project.description}
#
# 'version': plugin's version
version=${project.version}
#
# 'name': the plugin name
name=${elasticsearch.plugin.name}
### mandatory elements for site plugins:
#
# 'site': set to true to indicate contents of the _site/
# directory in the root of the plugin should be served.
site=${elasticsearch.plugin.site}
#
### mandatory elements for jvm plugins :
#
# 'jvm': true if the 'classname' class should be loaded
# from jar files in the root directory of the plugin.
# Note that only jar files in the root directory are
# added to the classpath for the plugin! If you need
# other resources, package them into a resources jar.
jvm=${elasticsearch.plugin.jvm}
#
# 'classname': the name of the class to load, fully-qualified.
classname=${elasticsearch.plugin.classname}
#
# 'java.version' version of java the code is built against
# use the system property java.specification.version
# version string must be a sequence of nonnegative decimal integers
# separated by "."'s and may have leading zeros
java.version=${maven.compiler.target}
#
# 'elasticsearch.version' version of elasticsearch compiled against
# You will have to release a new version of the plugin for each new
# elasticsearch release. This version is checked when the plugin
# is loaded so Elasticsearch will refuse to start in the presence of
# plugins with the incorrect elasticsearch.version.
elasticsearch.version=${elasticsearch.version}
#
### deprecated elements for jvm plugins :
#
# 'isolated': true if the plugin should have its own classloader.
# passing false is deprecated, and only intended to support plugins
# that have hard dependencies against each other. If this is
# not specified, then the plugin is isolated by default.
isolated=${elasticsearch.plugin.isolated}
#
