install_version() with multiple repositories

[kenahoo]

Following up on my previous ticket #1107, this PR adds the ability for install_version() to search multiple CRAN[-like] repositories for a specific version of a package. Currently, if it finds a package in an early repository but it's not the requested version, it will fail - this PR lets it continue searching the next repositories.

From the new docs I wrote for it:

This function knows how to look in multiple CRAN-like package repositories, and in their \code{archive} directories, in order to find specific versions of the requested package.

The repositories are searched in the order specified by the \code{repos} argument. This enables teams to maintain multiple in-house repositories with different policies - for instance, one repo for development snapshots and one for official releases. A common setup would be to first search the official release repo, then the dev snapshot repo, then a public CRAN mirror.

Thanks.

[kenahoo]

I see this is failing the Travis & AppVeyor builds. It seems to be a version thing on 'testthat', does it have something to with the change I made?

[kenahoo]

Any thoughts on this?

[jimhester]

I think this looks like a nice improvement, thank you for working on it. We will take a closer look at it nearer to the next devtools release with some more comments.

I apologize for the delayed response!

[kenahoo]

Great, glad to hear it! Thanks for taking a look.

[kenahoo]

I just rebased this WRT master.

[hadley]

Would you be mind porting this to remotes? That's what devtools will use for installation in the future. We are working on breaking devtools up into smaller pieces to try and make development a bit more fluid.