CI: Add whitespace test to CI

Signed-off-by: Andreas Fuchs <andreas.fuchs@sit.fraunhofer.de>

diff --git a/.travis.yml b/.travis.yml
index 96e60f5..2fd62a2 100644
--- a/.travis.yml
+++ b/.travis.yml

@@ -84,6 +84,8 @@
   - ./bootstrap
 
 script:
+# Check for whitespace errors
+  - git diff --check $(git hash-object -t tree /dev/null)
 # short-circuit normal build if we've already done a coverity scan
   - |
     if [ "${COVERITY_SCAN_BRANCH}" == 1 ]; then