Remove implicit dependency on outdated guzzle/guzzle

[stucki]

This project depends on "satooshi/php-coveralls": "^1" which loads guzzle/guzzle. This project is abandoned and results in a warning when running composer update:
Package guzzle/guzzle is abandoned, you should avoid using it. Use guzzlehttp/guzzle instead.

Unfortunately, the developers at https://github.com/satooshi/php-coveralls/ seem unwilling to add a simple fix for this problem, see php-coveralls/php-coveralls#203.

I propose two solutions to fix the issue, choose what you prefer more:

• move the dependency to require-dev (because it's only needed for that)
• alternatively, change the dependency to "satooshi/php-coveralls": "dev-master"

Let me know if I should create a pull request for this.

[seboettg]

Hi, thank you for this issue. Feel free to create PR for this.
Thanks!

[stucki]

Which of the two approaches do you prefer?