jruby: fix stack overflow in jgem command.

In jruby 1.7.10 (possibly earlier) the script in bin/gem simply calls bin/jgem. The jruby formula overwrites jgem with gem in the 'prefix with j' paragraph. This way the jgem script end up calling itself infinitely causing a stack overflow crash.

Closes Homebrew/legacy-homebrew#26910.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/jruby.rb

@@ -9,11 +9,13 @@ def install
     # Remove Windows files
     rm Dir['bin/*.{bat,dll,exe}']
 
-    # Prefix a 'j' on some commands
     cd 'bin' do
-      Dir['*'].each do |file|
-        mv file, "j#{file}" unless file.match /^[j]/
-      end
+      # Prefix a 'j' on some commands to avoid clashing with other ruby installations
+      ['ast', 'rake', 'rdoc', 'ri', 'testrb'].each { |f| mv f, "j#{f}" }
+      # Delete some unnecessary command
+      #  gem is a wrapper script for jgem
+      #  irb is an identical copy of jirb
+      ['gem', 'irb'].each { |f| rm f }
     end
 
     # Only keep the OS X native libraries