#Java 7, 149 bytes

float[]c(int[]x){int b=1<<31,a=b-1,j=0,l=x.length;for(int i:x){a=i<a?i:a;b=i>b?i:b;}float[]r=new float[l];for(;j<l;r[j]=x[j++]-a)*1f/(b-a);return r;}

Ungolfed & test code:

Try it here.

import java.util.Arrays;
class M{
  static float[] c(int[] x){
    int b = Integer.MIN_VALUE,
        a = b-1, // In Java, Integer.MIN_VALUE - 1 = Integer.MAX_VALUE (and vice-versa)
        j = 0,
        l = x.length;
    for(int i : x){
      a = i < a ? i : a; // Determine min value of array
      b = i > b ? i : b; // Determine max value of array
    }
    float[] r = new float[l];
    for(; j < l; r[j] = (x[j++] - a) * 1f / (b-a));
    return r;
  }

  public static void main(String[] a){
    System.out.println(Arrays.toString(c(new int[]{ 5, -20, 30 })));
    System.out.println(Arrays.toString(c(new int[]{ 1, 2, 3, 4, 5 })));
    System.out.println(Arrays.toString(c(new int[]{ 0, 5, 100, 400 })));
  }
}

Output:

[0.5, 0.0, 1.0]
[0.0, 0.25, 0.5, 0.75, 1.0]
[0.0, 0.0125, 0.25, 1.0]

Java 7, 149 bytes

float[]c(int[]x){int b=1<<31,a=b-1,j=0,l=x.length;for(int i:x){a=i<a?i:a;b=i>b?i:b;}float[]r=new float[l];for(;j<l;r[j]=x[j++]-a)*1f/(b-a);return r;}

Ungolfed & test code:

Try it here.

import java.util.Arrays;
class M{
  static float[] c(int[] x){
    int b = Integer.MIN_VALUE,
        a = b-1, // In Java, Integer.MIN_VALUE - 1 = Integer.MAX_VALUE (and vice-versa)
        j = 0,
        l = x.length;
    for(int i : x){
      a = i < a ? i : a; // Determine min value of array
      b = i > b ? i : b; // Determine max value of array
    }
    float[] r = new float[l];
    for(; j < l; r[j] = (x[j++] - a) * 1f / (b-a));
    return r;
  }

  public static void main(String[] a){
    System.out.println(Arrays.toString(c(new int[]{ 5, -20, 30 })));
    System.out.println(Arrays.toString(c(new int[]{ 1, 2, 3, 4, 5 })));
    System.out.println(Arrays.toString(c(new int[]{ 0, 5, 100, 400 })));
  }
}

Output:

[0.5, 0.0, 1.0]
[0.0, 0.25, 0.5, 0.75, 1.0]
[0.0, 0.0125, 0.25, 1.0]