feat: automatic code style checking

[misantron]

Fixes

Closes #108

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Run code style authomatic checking (PSR-1, PSR-2) on TravisCI build
• Make it possible to run unit tests and CS check with Composer commands
• Some fixes in TravisCI project configuration

[codecov]

Codecov Report

Merging #109 into master will decrease coverage by 3.75%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   88.00%   84.24%   -3.76%     
==========================================
  Files           4        2       -2     
  Lines         175      165      -10     
==========================================
- Hits          154      139      -15     
- Misses         21       26       +5     

Impacted Files	Coverage Δ
lib/Client.php	81.56% <100.00%> (-3.76%)	⬇️
lib/Response.php	100.00% <100.00%> (ø)
lib/Exception/InvalidRequest.php
test/unit/MockClient.php

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[thinkingserious]

Hello @misantron,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[peter279k]

This PR looks good to me.
Some request chnages need to be cncerned.

[misantron]

@peter279k made fixes for ext-mbstring dependency and EOF in gitignore and phpunit.xml.dist

[thinkingserious]

Hello @misantron,

Thanks again for the PR!

We noticed that you have earned 3 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

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[misantron]

@thinkingserious sendgrid/sendgrid-php#762

[thinkingserious]

Hello @misantron,

Thanks again for the PR!

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[childish-sambino]

LGTM

Nice work!