Updating Twilio SDK v5.15

[alexowson]

Updating Twilio SDK version to 5.15
Fixes to Twilio SDK v5.15

[codecov]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage     82.87%   82.87%           
  Complexity       49       49           
=========================================
  Files             4        4           
  Lines           146      146           
=========================================
  Hits            121      121           
  Misses           25       25           

Impacted Files	Coverage Δ	Complexity Δ
src/TwilioMessageDriver.php	96.29% <100.00%> (ø)	20.00 <0.00> (ø)
src/TwilioVoiceDriver.php	96.15% <100.00%> (ø)	18.00 <0.00> (ø)

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[calebporzio]

I ran into this same problem. I forked this repo and merged this PR and everything works perfectly now. Thanks for this @alexowson!