BUG: fix travis ref

Travis is currently pointing to the wrong sample data commit (tag),
which results in numerous doctest and extest failures.
BLD: change cf standard name table

Current table contains a bug, this commit switches to an ealier
table before this bug was introduced.
Merge pull request #586 from cpelley/fix_sampledata_ref

fix travis ref to new sample data
MAINT: Optimise constructing ordered metadata

Currently cell objects comparisons are made which have much greater
expense to native python datatypes (tuples) when thousands of 'fields'
are concerned.  This commit switches to using standard tuples, resulting
in 50% gains in speed over the code segment.