Revert "TST: Use a "dev env" Q2 version in Travis #154"

This reverts commit 1d77e64.

Once QIIME 2 2019.7 is released, I'll revert this revert (going
back to testing with the dev version). However, the thing is -- the
dev version currently available has the Differentials type definition
merged in, which causes a conflict with songbird. So uhhh the best
thing to do is keep testing with 2019.4, then switch over once all
of the infrastructure is updated (and at this point we can say that
Qurro only supports QIIME 2019.7).

.travis.yml

@@ -14,12 +14,16 @@ before_install:
     - conda config --set always_yes yes --set changeps1 no
     - conda update -q conda
     - conda info -a
-    # 2. Install the latest QIIME 2 version
-    # (see https://dev.qiime2.org/latest/quickstart, thanks @thermokarst for
-    # the heads up :)
-    - wget https://raw.githubusercontent.com/qiime2/environment-files/master/latest/staging/qiime2-latest-py36-linux-conda.yml
-    - travis_retry conda env create -n qiime2-dev --file qiime2-latest-py36-linux-conda.yml
-    - source activate qiime2-dev
+    # 2. Install QIIME 2 2019.4 (commands from
+    # https://docs.qiime2.org/2019.4/install/native)
+    #
+    # With QIIME 2 2018.11, I get a version conflict with tensorflow (needed
+    # for songbird) and Q2's required numpy versions. We could probably get
+    # around that, but I'm ok with q2-qurro only officially supporting
+    # QIIME 2 2019.4 or later.
+    - wget https://data.qiime2.org/distro/core/qiime2-2019.4-py36-linux-conda.yml
+    - travis_retry conda env create -n qiime2-2019.4 --file qiime2-2019.4-py36-linux-conda.yml
+    - source activate qiime2-2019.4
     # 3. Install songbird (needed so QIIME 2 knows how to load
     # FeatureData[Differential] artifacts).
     - conda install -c conda-forge songbird