Add gem build autocompletion

plugins/gem/_gem

@@ -9,6 +9,7 @@ _gem_installed() {
 
 local -a _1st_arguments
 _1st_arguments=(
+  'build:Build a gem from a gemspec'
   'cert:Manage RubyGems certificates and signing settings'
   'check:Check installed gems'
   'cleanup:Clean up old versions of installed gems in the local repository'
@@ -53,6 +54,9 @@ if (( CURRENT == 1 )); then
 fi
 
 case "$words[1]" in
+  build)
+    _files -g "*.gemspec"
+    ;;
   list)
       if [[ "$state" == forms ]]; then
         _gem_installed