hotfix for gcc/4.8.x

[mhsieh]

ref:

1. -Wconversion warning nlohmann/json#259 (comment)
2. nlohmann/json@f86fe57

[mhsieh]

Don't merge just yet, we have not validated this.

[nlhepler]

https://github.com/nlohmann/json#supported-compilers

@mhsieh gcc/4.8.x is emphatically unsupported by the json author due to bugs in that version's implementation of the stdlib.

[pezmaster31]

That and this repo lives primarily in BB now, only mirrored here. We should place any PRs there, please.

[mhsieh]

I will remove the PR later, sorry about that.

[mhsieh]

@nlhepler - see that discussion thread, I think gcc/4.8.x might be still usable.

[nlhepler]

@mhsieh it does seem that way, but at the end of the day he vetoed it: nlohmann/json#212 (comment)

[mhsieh]

This will break the travis ci. /me sad panda