Make Gemfile env var relative to work locally

Signed-off-by: Arnaud J Le Hors <lehors@us.ibm.com>

Author: lehors

netlify.toml

@@ -4,7 +4,7 @@
   command = "./combine-versions.sh && bundle exec jekyll build"
 
   [build.environment]
-    BUNDLE_GEMFILE = "/opt/build/repo/www/Gemfile"
+    BUNDLE_GEMFILE = "Gemfile"
 
 [context.deploy-preview]
   command = "./combine-versions.sh && bundle exec jekyll build --drafts --future"