Flexbox used instead of fixed width.

[yasemincidem]

The component made work with also relative width (flex etc..)
For example:

<ProgressBar
          fillStyle={{}}
          backgroundStyle={{backgroundColor: '#cccccc', borderRadius: 2}}
          style={{marginTop: 10, width: 300}}
          progress={this.state.progress}
  />

if we change the width:300 to flex:1 at above sample as shown below, progress-bar will not work because there is no fixed width property on style prop.

<ProgressBar
          fillStyle={{}}
          backgroundStyle={{backgroundColor: '#cccccc', borderRadius: 2}}
          style={{marginTop: 10, flex:1}}
          progress={this.state.progress}
      />

we can find the progress-bars width ourselves through code. this pr aims to made the plugin work with relative widths also. by detecting the width of progress programatically.