Do not update local jgo test unless -u or -U option specified

[hanslovsky]

Fixes #27

For comparison, run this script:

#/usr/bin/env sh

if [ -z "${CONDA_PREFIX}" ]; then
    export PATH=$HOME/.local/bin:$PATH
fi

echo "Time with update"
time jgo -u org.janelia.saalfeldlab:paintera:0.8.2 --version 2>/dev/null
echo
echo
echo "Time without update"
time jgo org.janelia.saalfeldlab:paintera:0.8.2 --version 2>/dev/null

On latest conda-forge release:

Time with update

real    0m1.956s
user    0m5.883s
sys     0m0.240s


Time without update

real    0m1.912s
user    0m5.728s
sys     0m0.275s

On this PR branch:

Time with update

real    0m1.846s
user    0m5.401s
sys     0m0.270s


Time without update

real    0m0.369s
user    0m0.550s
sys     0m0.070s