Remove extra whitespace between version numbers via --iversions

[rasbt]

Remove the whitespaces in --iversion's output so that it's consistent with other output. E.g.,

%watermark -iv -v -w 
dask        0.15.1
CPython 3.6.1
IPython 6.1.0
watermark 1.5.0

should be

%watermark -iv -v -w 
dask 0.15.1
CPython 3.6.1
IPython 6.1.0
watermark 1.5.0

Alternatively, everything could be aligned, e.g., say version numbers starting in the 20th column (I don't any python package is more than 18 characters long ;)