[travis-ci] Only run linters on Pull Requests

Linters should only be run on pull request builds. Once bad code style
has been merged into master, it's too late. Master and other branches
should not fail to build because of linter warnings.

.travis.yml

@@ -52,7 +52,7 @@ before_script:
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/git-subtree-check.sh src/leveldb; fi
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/check-doc.py; fi
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/check-rpc-mappings.py .; fi
-    - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/lint-all.sh; fi
+    - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_EVENT_TYPE" = "pull_request" ]; then contrib/devtools/lint-all.sh; fi
     - unset CC; unset CXX
     - mkdir -p depends/SDKs depends/sdk-sources
     - if [ -n "$OSX_SDK" -a ! -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then curl --location --fail $SDK_URL/MacOSX${OSX_SDK}.sdk.tar.gz -o depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi