tools: travis: skip the build if only markdown files are changed

Credit: tritoch at https://github.com/travis-ci/travis-ci/issues/6301#issuecomment-313559284

.travis-maybe-skip.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+# Maybe skip (terminate) this travis build, if it seems unnecessary.
+# Emulates appveyor's more powerful skip/only feature.
+
+set -e
+
+# works only on master ? 
+CHANGED_FILES=`git diff --name-only master...${TRAVIS_COMMIT}`
+
+# only files not matching this pattern will trigger a build
+# extended regex, don't quote regex metachars
+#SKIP_PAT="(^site/con|do.nload)"
+SKIP_PAT="\.md$"
+
+# doesn't handle empty CHANGED_FILES correctly, shouldn't matter
+SKIP=True
+for F in $CHANGED_FILES; do
+  if ! [[ $F =~ $SKIP_PAT ]]; then
+    SKIP=False
+    break
+  fi
+done
+
+if [[ $SKIP == True ]]; then
+  echo "Only skippable files found, exiting."
+  travis_terminate 0
+  exit 1
+else
+  echo "Non-skippable files found, continuing with build."
+fi

.travis.yml

@@ -71,6 +71,9 @@ branches:
 #   - ghc: 7.10.3
 #   - ghc: 8.0.2
 
+before_install:
+  - ./.travis-maybe-skip.sh
+
 install: # command output is hidden as they complete
 
   # if any command fails, end the travis script