Allow Guzzle 7

[Nyholm]

We just released a beta of Guzzle 7 today. The BC breaking changes have been kept to a minimum.

This PR allows users to use AWS SDK with Guzzle 7.

[codecov-io]

Codecov Report

Merging #1940 into master will decrease coverage by 0.08%.
The diff coverage is 36.36%.

@@             Coverage Diff              @@
##             master    #1940      +/-   ##
============================================
- Coverage     93.33%   93.25%   -0.09%     
  Complexity     3504     3504              
============================================
  Files           195      195              
  Lines          9307     9319      +12     
============================================
+ Hits           8687     8690       +3     
- Misses          620      629       +9

Impacted Files	Coverage Δ	Complexity Δ
src/functions.php	72.22% <36.36%> (-4.3%)	0 <0> (ø)

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[howardlopez]

Thanks for the PR! Just a couple comments, and also can you add a changelog?

[Nyholm]

I updated the PR with Travis config, changelog and a cache.

Im hesitant the cache is needed to be honest. I ran a small test:

Without cache: 0,018428ms
With cache:    0,006540ms

With cache is indeed much faster. It runs in 35% of the time. But this is running 10 million times! So I think it is fair to go without a cache. =)

Especially considering that we are often doing a HTTP call after calling this function.

[Nyholm]

Thank you for merging.