Define correct isnan, isinf functions

[janza]

fixes #520

[janza]

This PR fixes building node-canvas on linux with io.js. I've also added travis io.js target to confirm that it's working.

[kangax]

/cc @woodcoder @TooTallNate @rvagg

[rvagg]

Using an io.js branch of dnt:

~/git/node-canvas $ dnt
node@v0.11.16: PASS wrote output to /tmp/canvas-dnt-v0.11.16.out
node@v0.10.36: PASS wrote output to /tmp/canvas-dnt-v0.10.36.out
node@v0.12.0 : PASS wrote output to /tmp/canvas-dnt-v0.12.0.out
iojs@v1.1.0  : FAIL wrote output to /tmp/canvas-dnt-v1.1.0.out
iojs@v1.0.0  : FAIL wrote output to /tmp/canvas-dnt-v1.0.0.out
iojs@v1.2.0  : FAIL wrote output to /tmp/canvas-dnt-v1.2.0.out
Took 462s to run 6 versions of Node
[rvagg@fletcher (master) ~/git/node-canvas]$ curl https://github.com/Automattic/node-canvas/pull/522.patch | git am
Applying: define correct isnan, isinf functions
Applying: run travis tests against io.js
~/git/node-canvas $ dnt
node@v0.10.36: PASS wrote output to /tmp/canvas-dnt-v0.10.36.out
node@v0.11.16: PASS wrote output to /tmp/canvas-dnt-v0.11.16.out
node@v0.12.0 : PASS wrote output to /tmp/canvas-dnt-v0.12.0.out
iojs@v1.1.0  : PASS wrote output to /tmp/canvas-dnt-v1.1.0.out
iojs@v1.0.0  : PASS wrote output to /tmp/canvas-dnt-v1.0.0.out
iojs@v1.2.0  : PASS wrote output to /tmp/canvas-dnt-v1.2.0.out
Took 493s to run 6 versions of Node

so 👍 from me!

[jomo]

Pulled and tested (dokku & travis), issue indeed fixed.
👍 from me as well

[kangax]

Great, thanks everyone, merging!