obfuscate: use strings.Builder from go1.10

[gbbr]

This change updates JSON and Redis obfuscator to use strings.Builder
instead of bytes.Buffer for a minor speed increase. It also updates CI
to use go1.10, where this new structure was added.

Additionally, it also fixes a minor bug with an error message in the config
package.

[gbbr]

name                      old time/op    new time/op    delta
RedisObfuscator/445b-4      8.42µs ± 0%    6.48µs ± 0%  -23.06%
RedisQuantizer-4            1.27µs ± 0%    1.23µs ± 0%   -2.92%

name                      old alloc/op   new alloc/op   delta
RedisObfuscator/445b-4      9.68kB ± 0%    3.74kB ± 0%  -61.32%
RedisQuantizer-4            1.48kB ± 0%    1.37kB ± 0%   -7.32%

name                      old allocs/op  new allocs/op  delta
RedisObfuscator/445b-4         133 ± 0%        94 ± 0%  -29.32%
RedisQuantizer-4              12.0 ± 0%      11.0 ± 0%   -8.33%

[rochdev]

The documentation still mentions Go 1.9+ as the minimum version for development.

[gbbr]

@rochdev 👏 excellent! Done.