Add Psalm to build checks and fix issues

[muglug]

This PR adds checks by https://github.com/vimeo/psalm to the Travis build.

Fixes:

• a call to array_key_exists($fieldName, $resolver) on a value that possibly could have been an ArrayAccess. I believe array_key_exists always returns false when it isn't passed an array
• Ratchet\WebSocket\WsServer::disableVersion doesn't seem to exist
• Siler\Diactoros\emit's second argument had no effect, so I removed it

This uses dev-master because of some changes I added to Psalm to get it to understand composer autoload files.

[codecov]

Codecov Report

Merging #77 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master      #77     +/-   ##
===========================================
+ Coverage     96.82%   97.02%   +0.2%     
  Complexity       41       41             
===========================================
  Files            19       19             
  Lines           472      471      -1     
===========================================
  Hits            457      457             
+ Misses           15       14      -1

Impacted Files	Coverage Δ	Complexity Δ
src/Siler.php	100% <ø> (ø)	0 <0> (ø)	⬇️
src/Graphql/SubscriptionManager.php	100% <ø> (ø)	15 <0> (ø)	⬇️
src/Twig/Twig.php	100% <ø> (ø)	0 <0> (ø)	⬇️
src/Http/Http.php	100% <ø> (ø)	0 <0> (ø)	⬇️
src/Http/Response.php	100% <ø> (ø)	0 <0> (ø)	⬇️
src/Jwt/Jwt.php	100% <100%> (ø)	0 <0> (ø)	⬇️
src/Diactoros/Diactoros.php	100% <100%> (ø)	0 <0> (ø)	⬇️
src/Graphql/Graphql.php	81.81% <100%> (+1.04%)	0 <0> (ø)	⬇️
src/Http/Request.php	100% <100%> (ø)	0 <0> (ø)	⬇️

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[muglug]

Updated to undo const migration, after I added support for such in Psalm.

[leocavalcante]

Hi @muglug
Thanks for the contribution. Is this a replacement for PHPStan? Looks cool!
Can you fix code style issues? https://styleci.io/analyses/8AxpRl Please.
Cheers.

[muglug]

Is this a replacement for PHPStan

Certainly not a drop-in replacement. It's an alternative, started around the same time, with a slightly different approach.

[muglug]

Can you fix code style issues?

Fixed in 76a6c07!