Add phpstan to travis

[tspencer244]

• Use a docker container in travis to run phpstan
• Implement travis stages to run phpstan and generate code coverage once
• Closes #66

[codecov[bot]]

Codecov Report

Merging #76 into master will increase coverage by 1.6%. The diff coverage is n/a.

@@             Coverage Diff             @@
##             master      #76     +/-   ##
===========================================
+ Coverage     90.38%   91.99%   +1.6%     
  Complexity      175      175             
===========================================
  Files             9        9             
  Lines           468      437     -31     
===========================================
- Hits            423      402     -21     
+ Misses           45       35     -10

Impacted Files	Coverage Δ	Complexity Δ
src/Parser.php	100% <0%> (ø)	91% <0%> (ø)	:arrow_down:
src/Exception/ParserException.php	0% <0%> (ø)	2% <0%> (ø)	:arrow_down:
src/Processor/NamesProcessor.php	76.63% <0%> (+0.8%)	43% <0%> (ø)	:arrow_down:
src/Processor/LatexToUnicodeProcessor.php	73.91% <0%> (+3.54%)	10% <0%> (ø)	:arrow_down:

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[renanbr]

What elegance. Can you run a squash before I merge it?

[tspencer244]

What elegance. Can you run a squash before I merge it?

I have squashed it now

[renanbr]

thank you @tspencer244