Be specific about base version to be used

Toa void issues with `1.8.7-head` (that is triggered by RVM), we should be locking the version used for base to match the one installed.

Author: luislavena

bin/package_win32_fat_binary

@@ -12,7 +12,7 @@ cd '/vagrant'
 # passed in path of gem to be cross-compiled
 cd$1
 
-base_version=${BASE_VERSION:-1.8.7}
+base_version=${BASE_VERSION:-1.8.7-p374}
 cc_versions=${RUBY_CC_VERSION:-1.8.7:1.9.3:2.0.0:2.1.3}
 
 # need to use 1.8.7 for fat-binaries (1.9.3 can't cross-build 1.8.7)