Fix accession requirement in mirBASE dat file parser

[greatfireball]

Fix 134

Changes proposed within this pull request

the mirBASE dat parser does not require an accession field any more @microPIECE-team/micropiece-team-admins

[coveralls]

Pull Request Test Coverage Report for Build 391

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 91.732%

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Totals
Change from base Build 389:	0.006%
Covered Lines:	1176
Relevant Lines:	1282

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💛 - Coveralls