Use explicit pip install to install PyJulia from test.pypi.org

This is better because:

* We can make sure that the version installed in this environment is
  the one just uploaded (and fail if it cannot be found).
  See: https://travis-ci.org/JuliaPy/pyjulia/jobs/523283897#L448

* It doesn't rely on deprecated `indexserver` option.

* Simpler.

Author: tkf

ci/test-upload/tox.ini

@@ -1,14 +1,9 @@
 [tox]
 envlist = py
-indexserver =
-    test = https://test.pypi.org/simple/
 skipsdist = True
 
 [testenv]
-pip_pre = true
 deps =
-    :test:julia
-
     # These are the packages listed in extras_require in setup.py.
     # Not using `julia[test]` to avoid installing the test
     # dependencies from `test.pypi.org`:
@@ -18,6 +13,8 @@ deps =
     mock
 
 commands =
+    pip install --index-url https://test.pypi.org/simple/ julia==0.4.1.dev
+
     python -c "from julia import install; install()"
     python -m julia.runtests -- \
         --log-file {envlogdir}/pytest.log \