Update default API version to 1.39 in docs

[wxh06]

@thaJeztah #2512 b4beaaa

BTW, Travis CI's pre-installed Docker server version 18.06.0-ce does not support API v1.39 but 1.38.
So I suggest changing the default to 1.38 or auto.

Build system information
docker version
Client:
 Version:           18.06.0-ce
 API version:       1.38
 Go version:        go1.10.3
 Git commit:        0ffa825
 Built:             Wed Jul 18 19:11:02 2018
 OS/Arch:           linux/amd64
 Experimental:      false
Server:
 Engine:
  Version:          18.06.0-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       0ffa825
  Built:            Wed Jul 18 19:09:05 2018
  OS/Arch:          linux/amd64
  Experimental:     false

[thaJeztah]

Looks like this is a duplicate of #2640 ?

BTW, Travis CI's pre-installed Docker server version 18.06.0-ce does not support API v1.39 but 1.38.

Do they have a public issue tracker? Docker 18.06.0 reached EOL two years ago, and has known vulnerabilities; they really, really shouldn't be using that version anymore

[wxh06]

@thaJeztah
Do they have a public issue tracker?

Travis CI - Help Center § Get help from our support engineers

[milas]

Closing - this was changed to auto as of #2650