Updated MultiQC to v1.8 to solve problem with FastQC heatmaps

[skrakau]

With MultiQC 1.7 there are problems when running FastQC multiple times, i.e. the heat map for "per base sequence content" is missing for the second time. The problem was solved in v1.8. See also MultiQC/MultiQC#777

This will be useful when adding FastQC after the adapter trimming in #117.

nf-core/sarek pull request

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