Remove build targets

[Arcterus]

We can probably remove the 1.26.2 targets on ARM, as if they work on x86_64 they'll likely work on ARM (we don't do much architecture-specific stuff at the moment). We can also probably combine the formatting target with the normal stable Linux target.

[mssun]

We use matrix to create multiple targets. We can do that to reduce/combine some overlapped targets. However, the travis.yml file will become lengthy (to specify targets explicitly).

[Arcterus]

I think it's worth making the file more complex if the builds will be faster.

[mssun]

Fixed in the commit: 679459a