Update libc dependency version.
Update libc dependency version.

I changed this to 0.1.5 in the past when it should've always been
something in 0.2.* -- my mistake. 0.2.0 works fine so this is the
minimum required version.

mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-fitsio