travis-ci: verify different msgpack library versions

Also bump msgpack PyPI package name in requirements.txt. See the note
about the library name change in [1]:

 | Package name on PyPI was changed to msgpack from 0.5.

[1]: https://github.com/msgpack/msgpack-python/blob/8fb709f2e0438862020d8810fa70a81fb5dac7d4/README.md

Follows up #155

Author: Totktonada

Diff for: .travis.yml

@@ -8,6 +8,15 @@ cache: false
 env:
     matrix:
       - TARGET=test
+        PYTHON_MSGPACK=msgpack-python==0.4.0
+      - TARGET=test
+        PYTHON_MSGPACK=msgpack==0.5.0
+      - TARGET=test
+        PYTHON_MSGPACK=msgpack==0.6.0
+      - TARGET=test
+        PYTHON_MSGPACK=msgpack==0.6.2
+      - TARGET=test
+        PYTHON_MSGPACK=msgpack==1.0.0
       - OS=el DIST=6
       - OS=el DIST=7
       - OS=fedora DIST=28

Diff for: requirements.txt

@@ -1 +1 @@
-msgpack-python>=0.4.0
+msgpack>=0.4.0

Diff for: test.sh

@@ -9,8 +9,13 @@ echo "deb http://download.tarantool.org/tarantool/2x/ubuntu/ ${release} main" |
 sudo apt-get update > /dev/null
 sudo apt-get -q -y install tarantool
 
+# Install module requirements.
+#
+# Keep it in sync with requirements.txt.
+pip install "${PYTHON_MSGPACK:-msgpack==1.0.0}"
+python -c 'import msgpack; print(msgpack.version)'
+
 # Install testing dependencies.
-pip install -r requirements.txt
 pip install pyyaml
 
 # Run tests.