Fix a problem with gems bundled from a git repository

This occurred when the `bin/spring` was generated before 1.3.0.

Upgrading to 1.3.0 then lead to a bundler error. This happened because
in 1.3.0 we switched from forking the server process to spawning it as a
fresh process. In the fresh process, bundler didn't seem to like
GEM_HOME being set to an empty string.

We have already separately changed the generated binstub so that
GEM_HOME is nil, rather than the empty string
(666db7d). Therefore this fix just maps
the empty string from older binstubs to a nil value.

Author: jonleighton

CHANGELOG.md

@@ -1,3 +1,8 @@
+## Next release
+
+* Fix a problem with gems bundled from a git repository, where the
+  `bin/spring` was generated before 1.3.0.
+
 ## 1.3.0
 
 * Automatically restart spring after new commands are added. This means

lib/spring/client/run.rb

@@ -64,7 +64,10 @@ def run
       def boot_server
         env.socket_path.unlink if env.socket_path.exist?
 
+        # The GEM_HOME handling is to work around a problem with spring binstubs
+        # generated prior to 1.3.0.
         pid = Process.spawn(
+          ENV["GEM_HOME"] == "" ? { "GEM_HOME" => nil } : {},
           "ruby",
           "-r", "spring/server",
           "-e", "Spring::Server.boot"