Disable indent-tabs-mode in kivy-mode.el.

Summary: This fix sets indent-tabs-mode to nil so that indenting only uses spaces, instead of a combination of spaces and tabs.

I started with the following text in a buffer linked to a .kv file:
```
#:kivy 1.0.9
<MyRootWidget>:
    canvas:
Rectangle:
```
I put my cursor at the beginning of the Rectangle line, and pressed tab, expecting 8 spaces to be inserted. Instead a tab was inserted. Pressing it again resulted in cycling between 4 spaces, 0 spaces, and back to a tab. Thank God for whoever decided to highlight tabs in red, nevertheless it was still confusing. I can't imagine a reason for wanting the tab key to actually produce a tab in .kv files.

The behavior is caused by kivy-indent-line calling the indent-to function, which relies on indent-tabs-mode. The variable indent-tabs-mode is buffer-local when set, so it won't affect buffers other than .kv ones.