Add support for manylinux wheels on non-x86 platforms

This commit adds config options and identifiers for building manylinux
wheels on non-x86 platforms. Based on what travis and manylinux2014
support this includes aarch64 (ARMv8), ppce64le (power pc 64bit little
endian), and the s390x platforms.

Fixes #269

.travis.yml

@@ -9,6 +9,30 @@ matrix:
       services: docker
       env: PYTHON=python
 
+    # Linux Python 3
+    - sudo: required
+      language: python
+      python: 3.5
+      services: docker
+      arch: arm64
+      env: PYTHON=python
+
+    # Linux Python 3
+    - sudo: required
+      language: python
+      python: 3.5
+      services: docker
+      arch: ppc64le
+      env: PYTHON=python
+
+    # Linux Python 3
+    - sudo: required
+      language: python
+      python: 3.5
+      services: docker
+      arch: s390x
+      env: PYTHON=python
+
     # macOS Python 3
     - os: osx
       env: PYTHON=python3