[C] Specify travis NPM version

With NPM v3.10.8 optional dependencies specified in shrinkwrap are
installed even when they shouldn't be. This cause Karma installation to
fail on Travis servers, which in turn leads to failing builds. The issue
is tracked here: npm/npm#14042

.travis.yml

@@ -26,7 +26,7 @@ cache: bundler
 
 script:
   - cd client
-  - npm install -g npm
+  - npm install -g npm@3.10.7
   - npm version
   - npm install
   - npm run test