Update add_custom_gene.py

[6EMNKEY]

Changed #!/usr/bin/python for #!/usr/bin/env python, because if it is left in that way it won't complete execution of the fusioncatcher-build.py script

sh: 1: add_custom_gene.py: not found

ERROR: Workflow execution failed at step 25 while executing:

add_custom_gene.py \
--organism homo_sapiens \
--output /media/sequentia/visitors/visitor3/fusioncatcher/data/

If add_custom_gene.py is executed as the command above outside of the script, the result is the following

bash: /media/sequentia/visitors/visitor3/fusioncatcher/bin/add_custom_gene.py: /usr/bin/python: bad interpreter: No such file or directory