Avoid adding extension again when looking for library from versioned name

[nalimilan]

Improve detection of extension by first skipping trailing numbers and dots.
Previously, looking for a library via its versioned name (including extension)
would try with the extension added twice if loading using the raw name failed.
This would mask any errors from that step (e.g. due to missing symbols),
printing instead the "file not found" error from the second attempt with the
extension added.

[nalimilan]

Made the test less strict to avoid relying on OS-dependent messages.

[tkelman]

does this cause any issues on mac? don't the numbers come before the .dylib there?

[nalimilan]

I confess I'm not familiar with how this works on Mac, but this shouldn't create any problems: this change only allows detecting the extension when it's followed by a mix of digits and dots. If the version number appears before the extension, then it was already detected correctly, and nothing is changed in that regard.

(At some point, we probably should add a generic API to require a specific version, so that Julia takes care of generating the library name in a system-independent fashion.)

[nalimilan]

Though I could make this conditional on the extension being .so. Thoughts?

[nalimilan]

@tkelman More comments?

[tkelman]

Hoping this won't break anything? It's been long enough since this last ran on CI that we should make sure it still passes on Travis. Won't bother with appveyor since the queue is really long there right now, but travis isn't bad.

[tkelman]

Looks like on linux the tests passed but the build complained due to old cmake from having a stale .travis.yml. On Mac the stale .travis.yml prevented the build from going through. Good enough to merge I think.