fix type conversion for checksums

[boegel]

@Caylo I noticed that the case of a single checksum being listed as [('md5', '...')] got broken with #1826, we'll need to fix that since this introduces a regression and breaks a handful of existing easyconfigs, e.g. https://github.com/hpcugent/easybuild-easyconfigs/blob/master/easybuild/easyconfigs/c/CUDA/CUDA-6.5.14.eb and https://github.com/hpcugent/easybuild-easyconfigs/blob/master/easybuild/easyconfigs/c/CAP3/CAP3-20071221-intel-x86_64.eb, resulting in errors like:

== 2016-07-09 22:09:04,289 easyblock.py:2519 INFO Running source_checksum_step step
== 2016-07-09 22:09:04,289 filetools.py:463 DEBUG Using blocksize 16777216 for calculating the checksum
== 2016-07-09 22:09:06,276 filetools.py:502 DEBUG Computed md5 checksum for /user/data/gent/vsc400/vsc40023/EasyBuild/sources/c/CUDA/cuda_6.5.14_linux_64.run: 90b1b8f77313600cc294d9271741f4da (correct checksum: md5)
== 2016-07-09 22:09:06,361 build_log.py:152 ERROR Checksum verification for /user/data/gent/vsc400/vsc40023/EasyBuild/sources/c/CUDA/cuda_6.5.14_linux_64.run using ['md5', '90b1b8f77313600cc294d9271741f4da'] failed. (at easybuild/framework/easyblock.py:1489 in checksum_step)

please fix? :)