Fix broken build (due to libmseed breaking ABI in v3)

[acarapetis]

The HEAD of https://github.com/iris-edu/libmseed now points to the new libmseed version 3, which has changed the ABI from version 2 (which is what miniseed-rs is clearly expecting to link with). This PR modifies build.rs to explicitly check out the libmseed branch 2.x, allowing linking to succeed.

[savage13]

I will merge this soon. Would it be worth the effort to upgrade this crate to the 3.x ABI?

[acarapetis]

I'm sure it would be useful to someone at some point. For my purposes I'm perfectly happy with 2.x.

Also, maybe I'm missing something but seemed like the functionality of this crate was limited to reading the first record from each file/buffer? I've added a couple structs to allow stream processing of entire files in https://github.com/acarapetis/miniseed-rs/tree/io-helpers. There are no tests or documentation yet, but I could add some next week if you're interested in merging that too.

[HerrMuellerluedenscheid]

Probably this should be kept in a branch 2.x rather than master.

[HerrMuellerluedenscheid]

I will merge this soon. Would it be worth the effort to upgrade this crate to the 3.x ABI?

See #3

[savage13]

I added the changes from acarapetic/miniseed-rd#99c264a to read and write multiple segments from miniseed files.