Install cram in home directory and only if tests are active

.travis.yml

@@ -90,7 +90,10 @@ before_install:
         brew unlink cmake
         brew install llvm cmake
       fi
-  - pip install cram
+  - |
+      if [ "${TESTS:-ON}" = "ON" ]; then
+        pip install --user cram
+      fi
   - export CXX=${CC/gcc/g++}; export CXX=${CXX/clang/clang++}
   - $CC --version
   - $CXX --version