(#158) (#166) Resolve issue loading bundler from gem installs

[scotje]

This changes the method for resolving the installation path of a user
managed bundler gem to make it not depend on bundler itself.

Also cleans up the management of subprocess PATH from SDK-319.

[scotje]

Validated that the PATH fix still works for packaged builds:

Before:

[|] Checking metadata (metadata.json)strace: Process 22550 attached
[pid 22548] execve("/root/testmod/bin/metadata-json-lint", ["/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */] <unfinished ...>
[pid 22550] +++ exited with 0 +++
[pid 22548] <... execve resumed> )      = 0
[pid 22548] execve("/opt/puppetlabs/bin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = -1 ENOENT (No such file or directory)
[pid 22548] execve("/usr/local/sbin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = -1 ENOENT (No such file or directory)
[pid 22548] execve("/usr/local/bin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = -1 ENOENT (No such file or directory)
[pid 22548] execve("/usr/sbin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = -1 ENOENT (No such file or directory)
[pid 22548] execve("/usr/bin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = 0
strace: Process 22551 attached

After:

[|] Checking metadata (metadata.json)strace: Process 22573 attached
[pid 22571] execve("/root/testmod/bin/metadata-json-lint", ["/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */] <unfinished ...>
[pid 22573] +++ exited with 0 +++
[pid 22571] <... execve resumed> )      = 0
[pid 22571] execve("/opt/puppetlabs/pdk/private/ruby/2.1.9/bin/ruby", ["ruby", "/root/testmod/bin/metadata-json-lint", "--format", "json", "/root/testmod/metadata.json"], [/* 22 vars */]) = 0
strace: Process 22574 attached