Only run markdown linter on linux

Because:

* There's no need to run it on more than one platform
* It doesn't work on windows

.travis.yml

@@ -24,7 +24,7 @@ before_install:
 
 install:
   - |
-    if [ -z ${INTEGRATION} ]; then
+    if [ -z ${INTEGRATION} ] && [ "$TRAVIS_OS_NAME" == "linux" ]; then
       . $HOME/.nvm/nvm.sh
       nvm install stable
       nvm use stable

ci/base-tests.sh

@@ -14,7 +14,9 @@ set -ex
 echo "Running clippy base tests"
 
 PATH=$PATH:./node_modules/.bin
+if [ "$TRAVIS_OS_NAME" == "linux" ]; then
   remark -f *.md > /dev/null
+fi
 # build clippy in debug mode and run tests
 cargo build --features debugging
 cargo test --features debugging