Bump up seq_file dependency

[bricoletc]

Currently we bundle a 2012-version of seq_file and also an old version of htslib (im not sure which version however).

If we bump up seq_file to latest version (I see a date of 2015 in seq_file.h on the repo), we could:

• Bundle only the two header files in seq_file, like gramtools does
• Remove the string_buffer dependency, as new seq_file does not require it (this is written here)
• Bump up htslib

@iqbal-lab how useful would this be?