REDO: Remove leading path separator (:) from GOPATH

If TM_GOPATH was not set, the resulting GOPATH was starting with
colon (i.e. ":$GOPATH"), which was causing troubles in go1.5

Reapply cherrypick of efa0658 from tg.
Previously attempted in cf4a250,
accidentally overwritten in 2653890.
Issue identified by tg in #17.

Author: tg

Support/gomate.rb

@@ -9,7 +9,7 @@
 
 # TextMate's special GOPATH used in .tm_properties files prepended to the environment's GOPATH
 ENV['GOPATH'] = (ENV.has_key?('TM_GOPATH') ? ENV['TM_GOPATH'] : '') +
-                (ENV.has_key?('GOPATH') ? ':' + ENV['GOPATH'] : '')
+                (ENV.has_key?('GOPATH') ? ':' + ENV['GOPATH'] : '').sub(/^:+/,'')
 
 # Call tool to determine gopath
 if ENV.has_key?('TM_GO_DYNAMIC_GOPATH')