Update sratom to 0.6.2.

Sratom 0.6.2

  * Various minor code cleanups

Sratom 0.6.0

  * Add sratom_set_env() for setting prefixes
  * Fix padding of constructed vectors (thanks Hanspeter Portner)
  * Support round-trip serialisation of relative paths
  * Support sequences with beat time stamps
  * Fix warnings when building with ISO C++ compilers
  * Upgrade to waf 1.8.14

(bsiegert)