Update build_app.sh to remove mobile platforms before building

[jt55401]

This should fix issue #51

[arunoda]

What if there is no platform in this app. I wish it's better if we could simply add following platforms to the .meteor/platforms file.

server
browser

[jshimko]

You could also clear out .meteor/platforms and replace the contents with server and browser. Perhaps something like:

# base/scripts/lib/build_app.sh

# clear out the file
> .meteor/platforms

# add new strings
cat <<EOF >> .meteor/platforms
server
browser
EOF

[MichaelJCole]

There is now a --server-only flag for this purpose.

meteor/meteor#6270