base: Fix building of OpenSSL. See #73

This commit fixes the following error reported when building the base
image.

sh: 1: [: =: unexpected operator
sh: 1: [: =: unexpected operator

It turns out that (1) DEFAULT_DOCKCROSS_IMAGE is not set when building the
base image and (2) latest openssl will not be available in linux-x64 and
linux-x86 images.

common.docker

@@ -10,8 +10,8 @@ WORKDIR /usr/src
 RUN wget https://www.openssl.org/source/openssl-1.0.2j.tar.gz && \
   tar -xzvf openssl-1.0.2j.tar.gz && \
   cd openssl-1.0.2j && \
-  WRAPPER=$( [ $DEFAULT_DOCKCROSS_IMAGE = "dockcross/manylinux-x86" ] && echo "linux32" || echo "") && \
-  CONFIG_FLAG=$( [ $DEFAULT_DOCKCROSS_IMAGE = "dockcross/manylinux-x86" ] && echo "-m32" || echo "") && \
+  WRAPPER=$( [ "$DEFAULT_DOCKCROSS_IMAGE" = "dockcross/manylinux-x86" ] && echo "linux32" || echo "") && \
+  CONFIG_FLAG=$( [ "$DEFAULT_DOCKCROSS_IMAGE" = "dockcross/manylinux-x86" ] && echo "-m32" || echo "") && \
   ${WRAPPER} ./config --prefix=/usr $CONFIG_FLAG && \
   ${WRAPPER} make && \
   ( \