Support 32 bit builds on travis

[jarifibrahim]

Badger tests fail on 32 bit systems. We've tried to fix them multiple times but the crashes keep resurfacing. See #1384, #1142, #953, #89 .

#1216 was an attempt to run travis builds on 32 bit systems but travis seems it's run i386 tests on a 64-bit host.
As mentioned in #1384 (comment), it might be possible to run badger builds on a 32 bit container on travis on a 64 bit host and we should do that.

Here's an example of how delve runs 32 bit tests on a container on travis

# 386 linux
jobs:
  include:
    -  os: linux
       services: docker
       env: go_32_version=1.14

script: >-
    if [ $TRAVIS_OS_NAME = "linux" ] && [ $go_32_version ]; then
      docker pull i386/centos:7;
      docker run \
      -v $(pwd):/delve \
      --env TRAVIS=true \
      --env CI=true \
      --privileged i386/centos:7 \
      /bin/bash -c "set -x && \
           cd delve && \
           yum -y update && yum -y upgrade && \
           yum -y install wget make git gcc && \
           wget -q https://dl.google.com/go/go${go_32_version}.linux-386.tar.gz && \
           tar -C /usr/local -xzf go${go_32_version}.linux-386.tar.gz && \
           export PATH=$PATH:/usr/local/go/bin && \
           go version && \
           uname -a && \
           make test";
    else
      make test;
    fi

Taken from https://github.com/chainhelen/delve/blob/master/.travis.yml#L51-L82