WebP Support

[pascalknecht]

New version of #1638.
Improvements:

• skip image conversion if imagewebp is not defined (can happen on some servers)
• skip webp tests on travis

[coveralls]

Coverage decreased (-0.6%) to 93.657% when pulling 576523e on webkinder:webkinder_webp into bad57ac on timber:master.

[codecov]

Codecov Report

Merging #1777 into master will decrease coverage by 0.65%.
The diff coverage is 3.84%.

@@             Coverage Diff              @@
##             master    #1777      +/-   ##
============================================
- Coverage     94.91%   94.26%   -0.66%     
- Complexity     1526     1535       +9     
============================================
  Files            48       49       +1     
  Lines          3581     3607      +26     
============================================
+ Hits           3399     3400       +1     
- Misses          182      207      +25

Impacted Files	Coverage Δ	Complexity Δ
lib/ImageHelper.php	96.18% <0%> (-1.12%)	85 <1> (+1)
lib/Image/Operation/ToWebp.php	0% <0%> (ø)	8 <8> (?)
lib/Twig.php	99.43% <100%> (ø)	83 <0> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bad57ac...576523e. Read the comment docs.

[pascalknecht]

@jarednova What do I need to do to get this merged? It is clear that the code coverage drops if I don't run the unit tests of the WebP Class as it is not supported. Only way to support it would be to install ImageMagick and that would probably further slow down the tests. Any opinion?

[jarednova]

Thanks @pascalknecht for bringing this to the finish line. I made a few small tweaks in #1780 based on what you submitted...

Removed the unlinks —. Even though this is "neater", it's helpful to manually see the files generated during the tests to further verify the correct output. If we're going to do the unlinks (which a future 🌲 god might believe) lets apply it across the test suite: PNGs, JPGs and GIFs too)
While I didn't fix the imagewebp function problem, I did update the imagick install for Travis. Did it fix anything? No. But now it's up-to-date at least
Finally, I verified all the tests locally and feel comfortable ignoring the webp stuff from code coverage
Again, thanks to you and @mhz-tamb for making this happen!