test building the documentation on travis as well

.travis.yml

@@ -5,7 +5,7 @@ os:
   - osx
 
 env:
-  - variant=test_release lang=cpp11 sim=0 coverage=1
+  - variant=test_release lang=cpp11 sim=0 coverage=1 docs=1
   - variant=test_debug lang=cpp11 sim=1 coverage=0
   - variant=test_debug lang=cpp98 sim=0 coverage=0
   - variant=test_barebones lang=cpp11 sim=0 coverage=0
@@ -36,6 +36,7 @@ addons:
     - libboost1.55-tools-dev
     - python2.7-dev
     - g++-4.8
+    - ditaa
 
 before_install:
   - git submodule update --init --recursive
@@ -68,6 +69,7 @@ install:
   - ccache -V && ccache --show-stats && ccache --zero-stats
 
 script:
+  - if [ $docs == "1" ]; then cd docs; make; cd ..; fi
   - cd test
   - bjam --hash -j3 warnings-as-errors=on variant=$variant -l900 $coverage_toolset $target
 # if we're building with code coverage, report it as soon as possible