Fix icns generation

[mifi]

See #42.
Don't know why retina icons (ic11-14) breaks the icns file, but when I remove them it's working.

[mifi]

Travis CI build failed. I have now also fixed version of svg2png dependency to 3.1.0 because 3.1.1 introduced the breaking change of no longer supporting node 4. This can be reverted when svg2png releases a fix of the breaking change or if npm-icon-gen starts requiring minimum node 6.
See also domenic/svg2png#70

[akabekobeko]

@mifi

Thank you PR.

By the way, in the test of Travis CI has been under consideration to be in the Node version v6 and v7, or it is better that you also support v4?

[mifi]

In my opinion v4 should still be supported because lots of people still use it, and it's still official LTS. However it looks like domenic (author of svg2png) doesn't want to cooperate and fix his broken 3.1.1 version, so we have to either force v6 or set version of the svg2png dependency to =3.1.0

[akabekobeko]

@mifi

Thank you for the comment.

In order to apply the bugfix future of svg2png must be dropped Node v4 support.
For the svg2png equivalent of function is difficult to implement in this tool, Node v4 support gives up.

[mifi]

Yes, when v4 support is dropped you should publish a new major version