Remove Travis configuration and badge

[martinmoene]

Inspired by lest PR 74.

Actions:

• Delete .travis.yml
• Remove Travis badge from Readme.md

Plan:

• lest
• nonstd-lite-project
• any-lite
• atomic-lite
• bit-lite
• boolean-lite
• byte-lite
• expected-lite
• invoke-lite
• jthread-lite
• observer-ptr-lite
• optional-bare
• optional-lite
• ring-span-lite
• scope-lite
• span-lite
• status-value-lite
• string-view-lite
• type-lite
• value-ptr-lite
• variant-lite

Other:

• optional-fun-lite
• string-lite
• active-lite
• rpc-lite

[striezel]

The list above also contains invoke-lite, but I created no PR for that project, because the Travis CI badge has already been removed there: martinmoene/invoke-lite@68dce1b

[martinmoene]

Many thanks @striezel :)