Do not run travis build on forked repo (#41)

Author: liyanhui1228

.travis.yml

@@ -18,6 +18,8 @@ deploy:
   on:
     branch: master
 before_install:
-- openssl aes-256-cbc -K $encrypted_415d6cfcbdfb_key -iv $encrypted_415d6cfcbdfb_iv
-  -in python-compatibility-tools.json.enc -out python-compatibility-tools.json -d
-- export GOOGLE_APPLICATION_CREDENTIALS=python-compatibility-tools.json
+- if ! [ -z "$GOOGLE_APPLICATION_CREDENTIALS" ]; then
+    openssl aes-256-cbc -K $encrypted_415d6cfcbdfb_key -iv $encrypted_415d6cfcbdfb_iv
+    -in python-compatibility-tools.json.enc -out python-compatibility-tools.json -d;
+    export GOOGLE_APPLICATION_CREDENTIALS=python-compatibility-tools.json;
+  fi