Only run web3.js/explorer CI when targeting master branch

.travis.yml

@@ -67,7 +67,8 @@ jobs:
 
     # explorer pull request
     - name: "explorer"
-      if: type = pull_request
+      if: type = pull_request AND branch = master
+
       language: node_js
       node_js:
         - "node"
@@ -86,7 +87,8 @@ jobs:
 
     # web3.js pull request
     - name: "web3.js"
-      if: type = pull_request
+      if: type = pull_request AND branch = master
+
       language: node_js
       node_js:
         - "lts/*"