updated linux base and Blast+

[JWDebler]

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I had to update ubuntu base to 16.04 because the build process would otherwise crash while trying to install genometools. Also, Blast+ was updated to 2.7.1

[JWDebler]

Actually, you can ignore the linux base update in this Dockerfile, it is only necessary in the nf-repeatmasking file where genometools are installed. This one builds fine, so only update of Blast+ :-)