Retrieving LLVM version is now backwards compatible with Xcode 7

[ButkiewiczP]

This addresses issue #306 and fixes Slather for Xcode 7 builds

The main issue is line 239 (https://github.com/SlatherOrg/slather/blob/master/lib/slather/project.rb#L239) of project.rb, which parses the LLVM version from the output of xcrun llvm-cov --version. If the version can't be parsed, the match array is null, and the captures method fails. The stack trace returns:

/usr/local/lib/ruby/gems/2.4.0/gems/slather-2.4.1/lib/slather/project.rb:239:in `configure': undefined method `captures' for nil:NilClass (NoMethodError)
from /usr/local/lib/ruby/gems/2.4.0/gems/slather-2.4.1/lib/slather/command/coverage_command.rb:53:in `execute'
from /usr/local/lib/ruby/gems/2.4.0/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'
from /usr/local/lib/ruby/gems/2.4.0/gems/clamp-0.6.5/lib/clamp/subcommand/execution.rb:11:in `execute'
from /usr/local/lib/ruby/gems/2.4.0/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'
from /usr/local/lib/ruby/gems/2.4.0/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'
from /usr/local/lib/ruby/gems/2.4.0/gems/slather-2.4.1/bin/slather:17:in `<top (required)>'
from /usr/local/bin/slather:22:in `load'
from /usr/local/bin/slather:22:in `<main>'

When Xcode 7 is installed, or selected as the active Xcode app, running xcrun llvm-cov --version gives the following output:

bash-3.2$ xcrun llvm-cov --version
LLVM (http://llvm.org/):
LLVM version 7.3.0
Optimized build.
Built Apr  4 2016 (19:08:39).
Default target: x86_64-apple-darwin16.5.0
Host CPU: ivybridge`

When Xcode 8 is active, the command generates the following:

$ xcrun llvm-cov --version
Apple LLVM version 8.1.0 (clang-802.0.42)
Optimized build.
Default target: x86_64-apple-darwin16.5.0
Host CPU: skylake

The obvious difference here is the line containing the LLVM version information. Xcode 8 and beyond prefix this line with Apple, which is what the regex current checks for. The change to parse the LLVM version was only added 2 months ago, so Xcode 7 may have been an afterthought at the time. Ideally, there would be a better function to parse the version string, but simply removing the Apple prefix from the regex is good enough to fix the issue.

[coveralls]

Coverage remained the same at 95.648% when pulling c8d05f2 on IntrepidPursuits:pat/xcode7-support into 5361085 on SlatherOrg:master.

[ksuther]

Good find, thanks!