Only do coveralls for build all (#55)

We should probably turn off optimizations too.

Coveralls has a problem with source files which have no executable code in them. 
diff --git a/.travis.yml b/.travis.yml
index 51edb56..1343560 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,8 +8,11 @@
   - export LD_LIBRARY_PATH=$PWD/build/dist/lib:$LD_LIBRARY_PATH
 env:
   - USE_CONTEXT=ON
+    COVERALLS="-Dcoveralls_send=ON"
   - USE_CONTEXT=OFF
+    COVERALLS="-Dcoveralls_send=ON"
   - USE_CONTEXT=OFF USE_EMBEDTLS=ON
+    COVERALLS="-Dcoveralls_send=ON"
   - CMAKE_OPTIONS="-Dinclude_encrypt=ON -Dinclude_encrypt0=OFF -Dinclude_mac0=OFF -Dinclude_mac=OFF -Dinclude_sign=OFF -Dinclude_sign0=OFF"
   - CMAKE_OPTIONS="-Dinclude_encrypt=OFF -Dinclude_encrypt0=ON -Dinclude_mac0=OFF -Dinclude_mac=OFF -Dinclude_sign=OFF -Dinclude_sign0=OFF"
   - CMAKE_OPTIONS="-Dinclude_encrypt=OFF -Dinclude_encrypt0=OFF -Dinclude_mac0=ON -Dinclude_mac=OFF -Dinclude_sign=OFF -Dinclude_sign0=OFF"
@@ -37,7 +40,7 @@
   - cmake --version
   - git clone --depth 1 git://github.com/cose-wg/Examples Examples
   - mkdir build
-  - cd build && cmake -Duse_context=$USE_CONTEXT -Dcoveralls_send=ON  -Duse_embedtls=$USE_EMBEDTLS $CMAKE_OPTIONS .. && make all test
+  - cd build && cmake -Duse_context=$USE_CONTEXT -Duse_embedtls=$USE_EMBEDTLS $CMAKE_OPTIONS $COVERALLS .. && make all test
 
 after_success:
   - make coveralls