install nw-gyp in 32 bit environment if NODE_WEBKIT is not empty

Note: `make clean` happens after `npm install nw-gyp -g` because we intend to use `nw-gyp` later to rebuild the existing build of node-sqlite3.

.travis.yml

@@ -62,6 +62,8 @@ script:
 - npm test
 # publish 32 bit build
 - if [[ $PUBLISH == true ]]; then node-pre-gyp package publish; fi;
+# install nw-gyp in 32 bit environment if NODE_WEBKIT is not empty
+- if [ "$NODE_WEBKIT" != "" ]; then npm install nw-gyp -g; fi;
 - make clean
 # test source compile in 32 bit mode against external libsqlite3
 - if [[ "$platform" == 'linux' ]]; then sudo apt-get -y install libsqlite3-dev:i386; CC=gcc-4.6 CXX=g++-4.6 npm install --build-from-source --sqlite=/usr; npm test; fi