travis-build.sh: Use conda activate

[imagejan]

Newer versions of conda support activating an environment with conda activate $env, so let's use that recommended syntax instead of source activate $env.

I couldn't test this, but it hopefully fixes the Travis build failure here:
https://travis-ci.org/imagej/tutorials/builds/388369917

[ctrueden]

Worth a try.

[imagejan]

Hm, the build is still in error. Failing with:

CommandNotFoundError: Your shell has not been properly configured to use 'conda activate'.
If your shell is Bash or a Bourne variant, enable conda for the current user with
    $ echo ". /home/travis/miniconda/etc/profile.d/conda.sh" >> ~/.bashrc
or, for all users, enable conda with
    $ sudo ln -s /home/travis/miniconda/etc/profile.d/conda.sh /etc/profile.d/conda.sh
The options above will permanently enable the 'conda' command, but they do NOT
put conda's base (root) environment on PATH.  To do so, run
    $ conda activate
in your terminal, or to put the base environment on PATH permanently, run
    $ echo "conda activate" >> ~/.bashrc
Previous to conda 4.4, the recommended way to activate conda was to modify PATH in
your ~/.bashrc file.  You should manually remove the line that looks like
    export PATH="/home/travis/miniconda/bin:$PATH"
^^^ The above line should NO LONGER be in your ~/.bashrc file! ^^^
==> FAILED: EXIT CODE 1