Support LLVM 3.9 + 4.0

[TheDan64]

Description

• Added 4.0 test build to travis (3.9 has linking issues due to some travis-ubuntu issue, and may not be resolvable - will create an issue to do more research into it)
• Updated travis doc build to 4.0
• Added LLVM 3.9 & 4.0 support

Related Issue

Updates #1 to support LLVM 3.9 & 4.0

How This Has Been Tested

3.9 & 4.0 tests are passing. However, 3.9 is not being tested in travis due to a travis/ubuntu linking issue (should not affect most ubuntu users)

Option<Breaking Changes>

No breaking changes

Checklist

• I have read the Contributing Guide

[codecov]

Codecov Report

Merging #39 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   50.83%   50.85%   +0.01%     
==========================================
  Files          37       37              
  Lines        2939     2940       +1     
==========================================
+ Hits         1494     1495       +1     
  Misses       1445     1445

Impacted Files	Coverage Δ
src/passes.rs	97.89% <ø> (ø)	⬆️
src/values/metadata_value.rs	74.54% <ø> (ø)	⬆️
src/lib.rs	62.5% <ø> (ø)	⬆️
src/targets.rs	13.07% <ø> (ø)	⬆️
src/module.rs	58.15% <100%> (+0.22%)	⬆️

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