common.mk: clear envs for GEM

* common.mk: clear environment variables for GEM paths to get rid
  of searching unexpected gems.  fix failures on travis-ci.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51946 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -17,6 +17,10 @@ RUBYLIB       = $(PATH_SEPARATOR)
 RUBYOPT       = -
 RUN_OPTS      = --disable-gems
 
+GEM_HOME =
+GEM_PATH =
+GEM_VENDOR =
+
 SPEC_GIT_BASE = git://github.com/ruby
 MSPEC_GIT_URL = $(SPEC_GIT_BASE)/mspec.git
 RUBYSPEC_GIT_URL = $(SPEC_GIT_BASE)/rubyspec.git