chore(doc): remove unused doc folder

.travis.yml

@@ -35,7 +35,7 @@ after_success:
     tar xzf master.tar.gz && mkdir kcov-master/build && cd kcov-master/build && cmake .. && make && make install DESTDIR=../tmp && cd ../.. &&
     ls target/debug &&
     ./kcov-master/tmp/usr/local/bin/kcov --coveralls-id=$TRAVIS_JOB_ID --exclude-pattern=/.cargo target/kcov target/debug/hyper-*'
-  - '[ $TRAVIS_PULL_REQUEST = false ] &&
+  - '[ $TRAVIS_PULL_REQUEST = false ] && [ $TRAVIS_RUST_VERSION = stable ] &&
     { [ "$TRAVIS_TAG" != "" ] || [ "$TRAVIS_BRANCH" == "master" ]; } &&
     ./.travis/docs.sh'